Reverse a String Without Special Character

public class ReverseStringWithoutSpecialCharacter {

    public String reverse(String input) {
        char temp = 0;
        char[] ch = input.toCharArray();
        for (int i = 0, j = input.length() - 1; i < j; ) {
            if (isAlphaNumeric(String.valueOf(ch[i])) &&
                    isAlphaNumeric(String.valueOf(ch[j]))) {
                temp = ch[i];
                ch[i] = ch[j];
                ch[j] = temp;
                i++;
                j--;
            } else if (!isAlphaNumeric(String.valueOf(ch[i]))) {
                i++;
            } else if (!isAlphaNumeric(String.valueOf(ch[j]))) {
                j--;
            }
        }
        return String.valueOf(ch);
    }


    public boolean isAlphaNumeric(String s) {
        String pattern = "^[a-zA-Z0-9]*$";
        return s.matches(pattern);
    }

    public static void main(String[] args) {
        ReverseStringWithoutSpecialCharacter reverseStringWithoutSpecialCharacter = new ReverseStringWithoutSpecialCharacter();
        System.out.println(reverseStringWithoutSpecialCharacter.reverse("Basant@gmail.com"));
    }
}